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SubjectRe: [PATCH 16/17] TSC reset compensation
On 06/15/2010 02:27 PM, Marcelo Tosatti wrote:
> On Mon, Jun 14, 2010 at 09:34:18PM -1000, Zachary Amsden wrote:
>
>> Attempt to synchronize TSCs which are reset to the same value. In the
>> case of a reliable hardware TSC, we can just re-use the same offset, but
>> on non-reliable hardware, we can get closer by adjusting the offset to
>> match the elapsed time.
>>
>> Signed-off-by: Zachary Amsden<zamsden@redhat.com>
>> ---
>> arch/x86/kvm/x86.c | 34 ++++++++++++++++++++++++++++++++--
>> 1 files changed, 32 insertions(+), 2 deletions(-)
>>
>> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
>> index 8e836e9..cedb71f 100644
>> --- a/arch/x86/kvm/x86.c
>> +++ b/arch/x86/kvm/x86.c
>> @@ -937,14 +937,44 @@ static inline void kvm_request_guest_time_update(struct kvm_vcpu *v)
>> set_bit(KVM_REQ_CLOCK_SYNC,&v->requests);
>> }
>>
>> +static inline int kvm_tsc_reliable(void)
>> +{
>> + return (boot_cpu_has(X86_FEATURE_CONSTANT_TSC)&&
>> + boot_cpu_has(X86_FEATURE_NONSTOP_TSC)&&
>> + !check_tsc_unstable());
>> +}
>> +
>> void guest_write_tsc(struct kvm_vcpu *vcpu, u64 data)
>> {
>> struct kvm *kvm = vcpu->kvm;
>> - u64 offset;
>> + u64 offset, ns, elapsed;
>>
>> spin_lock(&kvm->arch.tsc_write_lock);
>> offset = data - native_read_tsc();
>> - kvm->arch.last_tsc_nsec = get_kernel_ns();
>> + ns = get_kernel_ns();
>> + elapsed = ns - kvm->arch.last_tsc_nsec;
>> +
>> + /*
>> + * Special case: identical write to TSC within 5 seconds of
>> + * another CPU is interpreted as an attempt to synchronize
>> + * (the 5 seconds is to accomodate host load / swapping).
>> + *
>> + * In that case, for a reliable TSC, we can match TSC offsets,
>> + * or make a best guest using kernel_ns value.
>> + */
>> + if (data == kvm->arch.last_tsc_write&& elapsed< 5 * NSEC_PER_SEC) {
>> + if (kvm_tsc_reliable()) {
>> + offset = kvm->arch.last_tsc_offset;
>> + pr_debug("kvm: matched tsc offset for %llu\n", data);
>> + } else {
>> + u64 tsc_delta = elapsed * __get_cpu_var(cpu_tsc_khz);
>> + tsc_delta = tsc_delta / USEC_PER_SEC;
>> + offset -= tsc_delta;
>> + pr_debug("kvm: adjusted tsc offset by %llu\n", tsc_delta);
>> + }
>> + ns = kvm->arch.last_tsc_nsec;
>> + }
>> + kvm->arch.last_tsc_nsec = ns;
>> kvm->arch.last_tsc_write = data;
>> kvm->arch.last_tsc_offset = offset;
>> kvm_x86_ops->write_tsc_offset(vcpu, offset);
>> --
>>
> Could extend this to handle migration.
>

Also, this could be extended to cover the kvmclock variables themselves;
then, if tsc is reliable, we need not ever recalibrate the kvmclock. In
fact, all VMs would have the same parameters for kvmclock in that case,
just with a different kvm->arch.kvmclock_offset.
Zach


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